Conventional approaches to generating multiple narrow beams of radiation, or pencil beams, with wavelengths in each beam being deviated by the same angle, with respect to an incident angle of a main beam, include the following.
In a mirror array approach multiple beams are generated in reflection. However, accurate deviation angles are difficult to achieve without individually adjusting the mirrors. If a small beam size is desired mirror adjustment becomes difficult. Additionally, the illuminating optics may obscure light reflected at small deviation angles.
In a prism array approach multiple beams are generated in transmission and beam deviation is accomplished with refraction. The accuracy of deviation is difficult to control, requiring extremely accurate wedge angles. The equality of deviation of multiple wavelengths is limited by the dispersion of a selected prism material and becomes more difficult to accomplish as additional wavelengths are introduced. Additionally, the dispersion of some materials is temperature-dependent.
In a Fresnel prism approach the functionality of the prism array is provided with an array of Fresnel prisms, which are typically manufactured by replication processes. However, material dispersion effects still limit the accuracy with which multiple wavelengths in each beam are deviated.
Another approach employs binary optics wherein an array of phase gratings, each comprising 2.sup.N discrete phase steps, where N is an integer, are made photolithographically. While the grating period can be made very accurately, different wavelengths are deviated by different angles, due to diffractive dispersion. Furthermore, a requirement for providing deviation angles near 10.degree., and the desirability of using semiconductor laser diode sources with wavelengths near 0.8 micrometers, results in required grating spatial frequencies on the order of 300 lines/mm. The fabrication of binary optics having such spatial frequencies, and N&gt;1, is presently both difficult and expensive to achieve. Also, a binary optic with two levels (N=1) is, at best, only 40% efficient in any one diffractive order.
Holograms have also been employed to provide multiple beams. However, holograms are generally expensive to produce and fabricate. Also, multiple color holograms further increase expense.
It is also noted that a conventional diffraction grating can deviate multiple wavelengths such that one diffraction order of each may coincide at the same angle. However, in that conventional diffraction gratings are usually blazed for the first diffraction order, this is an inefficient process, especially when attempting to provide higher diffraction orders of closely spaced wavelengths. That is, the resulting higher diffraction order optical signals may be at such low levels as to be unusable.
It is thus an object of the invention to overcome these problems and to provide method and apparatus that generates, in an efficient manner, multiple beams having equal deviation angles for multiple wavelengths.
It is another object of the invention to employ blazed diffraction gratings for creating multiple beams in an efficient manner.
It is a further object of the invention to employ blazed diffraction gratings for creating multiple sample beams for use in a multiple-wavelength interferometer.